UPDATE Tiempo
SET Tiempo.Estado = (select case when (sum (([VarAcumMensual].[IDF]+[VarAcumMensual].[IDV])-([VarAcumMensual].[GDF]
+[VarAcumMensual].[GDV])))>0 then 'Bº' else 'Pª' end from [VarAcumMensual] WHERE (((VarAcumMensual.IdTipoVariable)=2)
AND (([VarAcumMensual].Fecha) between '01/01/00' and '31/12/2002')) and VarAcumMensual.IdVAriable=Tiempo.IdVariable
GROUP BY VarAcumMensual.IdVariable );